Field effect transistors (FETs) using gallium arsenide (GaAs) have been widely put into practical use as semiconductor devices which have excellent high-frequency characteristics and which operate in micrometer/millimeter/submillimeter wavebands.
A conventional semiconductor device, for example, includes: a substrate; a gate electrode, a source electrode and a drain electrode, each of which includes multiple fingers and is provided on top of the substrate; a gate terminal electrode which is provided on top of the substrate and gathers all the fingers of the gate electrode; a source terminal electrode which is provided on top of the substrate and gathers all the fingers of the source electrode; and a drain terminal electrode which is provided on top of the substrate and gathers all the fingers of the drain electrode. The semiconductor device further includes: a backside metal electrode which is formed on the backside surface of the substrate; a through-hole which is formed in the substrate and connects the source terminal electrode to the backside metal electrode.
A semiconductor device that has a through-hole and a method of manufacturing the semiconductor device are known, for example, by JP, PH08-78437A.
In general, an FET and matching circuits to help the FET to achieve better performance are incorporated together in a package. The matching circuits are connected to both the gate electrode and the drain electrode of the FET by wire bonding using gold wires.
Aluminum (Al) is conventionally used for gate pads. Aluminum, however, exhibits poor adhesion to a GaAs substrate. In addition, an increase in a bonding force needed to bond gold wires causes a problem that the gate pad comes off the substrate.
Accordingly, an ohmic metal electrode is provided under the gate pad. The ohmic metal electrode reacts with the GaAs substrate to thus increase the adhesion of the ohmic metal electrode to the GaAs substrate.
When a negative voltage is applied to the gate electrode with the voltage of the backside metal electrode set at 0 V, an electric current flows between the ohmic metal electrode and the backside metal electrode through the substrate. In other words, an electric current flows between the gate electrode and the backside metal electrode. The flow of the electric current impairs the performance of the FET.